Metal base board and electronic equipment using the same

ABSTRACT

A metal base board comprising a metal plate, a circuit conductor section, and an insulating layer provided between the circuit conductor and the metal plate wherein the insulating layer comprises an organic insulating material with flaky inorganic fillers added therein and the flaky inorganic fillers are stacked in the insulating layer in a stratified state.

This is a continuation of application Ser. No. 08/482,854 filed Jun. 7,1995, now U.S. Pat. No. 5,578,366, which is a continuation ofapplication Ser. No. 08/285,699, filed Aug. 4, 1994, now abandoned.

FIELD OF THE INVENTION

This invention relates to a metal base board comprising a metallic basesection, a circuit conductor section, and an insulating section providedbetween said circuit conductor section and the base section, and moreparticularly a metal base board used in a semiconductor device for powercontrol which is incorporated in electronic equipment and generates aquantity of heat.

BACKGROUND OF THE INVENTION

In electronic control devices such as an invertor, servo amplifier, andspindle amplifier, a semiconductor device for power control compriseslarge capacity diodes, transistors, IGBTs, and MOSFETs. There is aso-called "DBC board" as a board for a circuit used in a electroniccircuit generating a large quantity of heat such as a power controlsection. The DBC board comprises a metal plate for a conductor circuit,a ceramic insulating material, and a heat-conductive metal plate joinedto a base of said ceramic insulating material. The metal plate used inthe DBC board, for instance may be a copper plate. The ceramicinsulating material used, for instance, may be alumina ceramic oralumina ceramic nitride. The heat-conductive metal plate, for instance,may be a copper plate. If such a material as alumina ceramic ornitriding alumina ceramic nitride, each with a high heat conductivity,is used as a ceramic insulating material, heat generated in a conductorcircuit is rapidly transferred to a metal plate. Furthermore, as thematerial itself is ceramic, degradation of the material due to coronadischarge is suppressed, and also the resistance against electricdischarge is improved.

In the DBC board, however, as a coefficient of thermal expansion of theceramic insulating material layer is different from that of the heatconductive metal plate layer, the heat cycle characteristic is ratherpoor, and cracks may be generated in the joining interface. Also, due torestrictions in the manufacturing process of ceramics, it is difficultto manufacture a large size plate. Typically, a ceramic product islimited to a dimension of around 150 mm×150 mm, using the conventionalmanufacturing method. In addition, the heating of materials at a hightemperature is required in a process of manufacturing the conventionalboard, and as a result, the manufacturing process is very complicatedwith a high production cost.

To solve the problems with the DBC board as described above, use of ametal base board in which an insulating layer made of organic polymerand metallic foil, and adhered to a heat-conductive metal plate with anadhesive, has been proposed. As shown in FIG. 7, in this metal baseplate 70, a circuit conductor 3 is formed via an insulating layer 2comprising an organic insulating material on the upper surface of theheat-conductive metal plate 1, in which a granular inorganic fillermaterial is added to an insulating layer 2. This configuration isdisclosed, for instance, in the Japanese Patent Publication No.6235/1971and the Japanese Patent Publication No.9650/1972. A discharge path 71 isseen between conductor 3 and plate 1.

Generally, organic polymer constituting an insulating layer has asubstantially lower heat conductivity as compared to that of ceramicsused in the DBC board, but the heat conductivity is raised by addingsuch an inorganic filler material as alumina or silica. Furthermore, theheat conductivity is raised to a desired level by reducing the thicknessof the insulating layer to around 7% to 30% of that of the DBC board.

The technological documents relating to this invention includes theJapanese utility Model Laid Open Publication No.106775/1991 disclosing a"metal base board", the Japanese Utility Model Laid Open PublicationNo.73966/1988 disclosing an "heat-radiating insulating board", theJapanese Utility Model Laid Open Publication No.98253/1987 disclosing a"printed-circuit board", the Japanese Patent Laid Open PublicationNo.244180/1991 disclosing a "Metal base laminated plate", the JapanesePatent Laid Open Publication No.27786/1990 disclosing a "low heatresistance circuit board", the Japanese Patent Laid Open PublicationNo.232795/1989 disclosing a "Method of Manufacturing a Metal BaseBoard", and the Japanese Patent Laid Open Publication No.232792/1990disclosing a "Circuit Board".

Specifically, FIG. 1 or FIG. 4 in the Japanese utility Model Laid OpenPublication No.106775/1991 disclose a metal base board, in which lengthyinorganic fillers having a high heat conductivity are added in anorganic insulating layer. The longitudinal surfaces thereof are facingthe metal base side or are disposed at random. Because the inorganicfillers are added, heat radiated from parts mounted to the circuitconductor 3 of a metal base board 70 such as transistors, resistors, andcapacitors, can effectively be transferred for emission to the outside.

Furthermore, as an organic polymeric insulating material is used inthese metal base boards, a board which is larger than a DBC board caneasily be manufactured with a low cost. Furthermore, there is no problemwith the formation of cracks due to a heat cycle, as in a DBC board. Forthe reasons as described above, a metal base board having theconfiguration as described above is employed in a power circuit or othersimilar electric components which generate a large quantity of heat.

In the conventional type of metal base board as described above,however, a granular inorganic filler material is added to an organicpolymeric material and the resultant mixture is used as an insulatingmaterial. However, the organic insulating layer is easily degraded whenexposed to electric discharge, and the granular filler material does notwork effectively. Namely, when degradation due to electric dischargeproceeds up to the section with the granular inorganic filler materialadded therein, the degradation will continue to proceed along thesection interface. As a result, the filler is not always effective inimproving the electric discharge resistance characteristics.

In addition, as the thickness of the insulating layer is reduced toapproximately 7% to 30% of the thickness of an ordinary DBC board toinsure a desirable heat radiating characteristics, the insulatingcharacteristics will be affected, under high voltage. First, it shouldbe noted that, if the thickness of an insulating layer is increased, theresistance against electric discharge is improved, but the heatradiating characteristics becomes lower because heat generated in thecircuit conductor is hardly transferred to the metal plate. Also, itshould be noted that, although a conventional type of metal base boardis generally used in applications requiring a relatively low operatingvoltage, i.e., AC 200 V or below, recently a metal base board is oftenused in applications where a high voltage in the class of 400 V or 600 Vis always applied. Thus, under these two conditions, the insulatinglayer made of organic polymer will be degraded due to corona electricdischarge, and the insulation, characteristics of the insulating layerwill be damaged. In addition, as the thickness of a plate is small, thelife until the final breakage of insulation will be short.

There are several reasons for this problem. First, if there is air on asurface along a conductor circuit section of a board, and even if thesection has been processed with, for instance, resist or a siliconpotting material, there exist void sections with air filled therein inthe processed layer. Second, it is known that, if a high AC voltage isapplied to such air-filled void sections, corona electric discharge isgenerated. Third, in most organic polymeric materials a voltage at whichcorona electric discharge starts is in a range of approximately 400 to500 V, although it depends on the thickness of each board. Not only inelectronic equipment having a rated voltage of 400 V or less, but alsoin electronic equipment for which a rated voltage is more than 400 V, ifa voltage of more than 400 V is loaded during its service operation inassociation with electronic control of such sections as a switch, coronaelectric discharge may occur.

Furthermore, although the "metal base board" disclosed in the JapaneseUtility Model Laid Open Publication No.106775/1991 is effective toprovide thermal advantages, such as improvement of heat conductivity andreduction of coefficient of linear thermal expansion, degradation due toelectric discharge will proceed along an interface, as previouslydescribed for the board shown in FIG. 7 with a granular inorganic fillermaterial added therein when a degradation due to electric dischargereaches the fillers. For this reason, the filler can not improve aresistance against electric discharge of a metal base board.

SUMMARY OF THE INVENTION

It is an object of the invention to provide metal base boards whichprovide excellent heat-radiating characteristics as well as excellentresistance against electric discharge, and can be used even under a highvoltage. It also is an object to provide electronic equipment which usessuch metal base board and operates at high voltage.

In a metal base board according to the present invention and in theelectronic equipment using the same, an insulating layer comprises anorganic insulating material with flaky inorganic fillers added therein.The flat faces of said flaky inorganic fillers are aligned basically inparallel with the surfaces of the circuit conductor section and the basesection so that the inorganic flaky filler plates are stacked in astratified state. With this arrangement, even if corona discharge isgenerated in a void at the surface of a circuit conductor section orwithin an insulating layer and the organic insulating material isdegraded, the degradation due to discharge is prevented or substantiallysuppressed in the section with flaky inorganic fillers added therein. Asa result, the discharge resistant characteristics of the entireinsulating layer are improved.

In a metal base board according to the present invention and theelectronic equipment using the same therein, the insulating sectioncomprises an organic insulating material with flaky inorganic fillersand a granular inorganic filler material, flat surfaces of the flakyinorganic fillers are aligned basically in parallel with surfaces of thecircuit conductor section and the base section so that the materials canbe stacked in a stratified state. As a result, even if discharge occursin a void along the surface of the circuit conductor or in a voidsection within the insulating layer and the organic insulating materialis degraded due to the discharge, the degradation due to dischargeprevented or substantially is suppressed in the section with the flakyinorganic fillers added therein. Also, the discharge resistantcharacteristics of the entire insulating layer are improved, and becauseof the granular inorganic filler material, the heat conductivity of theinsulating layer is raised. At the same time, the linear thermalexpansion coefficient can be reduced, so that an insulating layer suitedto conditions for practical use can be formed by changing a fixing ratioof the filler material.

In a metal base board according to the present invention and electronicequipment using the same, the insulating section comprises an insulatinglayer made of an organic insulating material with flaky inorganicfillers added therein and an insulating layer made of an organicinsulating material with a granular inorganic filler material addedtherein, namely two layers in all, the surfaces of the flaky inorganicfillers are aligned basically in parallel with the surfaces of thecircuit conductor and the base section so that the filler materials canbe stacked in a stratified state. As a result, even if the organicinsulating material is degraded due to electric discharge, thedegradation due to electric discharge is suppressed in the section withthe flaky inorganic fillers added therein, or the degradation due to theelectric discharge itself is prevented, and the electric dischargeresistant characteristics are improved. Further, because of theinsulating layer comprising a granular inorganic filler material, theheat conductivity of the insulating layer is raised and also the linearthermal expansion coefficient can be reduced. With this arrangement, aninsulating layer suited to conditions for practical use can be formed bychanging a ratio of thickness between the two organic insulating layers.

In the metal base board according to the present invention andelectronic equipment using the same, the insulating section comprises aplurality of insulating layers, each insulating layer comprises anorganic insulating layer with a different type of flaky inorganicfillers each having a different average particle diameter respectively.Fillers belonging to at least any one type of the flaky inorganic fillermaterials are aligned basically in parallel with the surfaces of thecircuit conductor and the base section so that the fillers can bestacked in a stratified state. As a result, even if electric dischargeoccurs in a section of the circuit conductor along the surface or in avoid section within the insulating layer and the organic insulatingmaterial is degraded due to the electric discharge, the degradation dueto electric discharge is suppressed in the section with the flakyinorganic fillers added therein, or the degradation due to electricdischarge itself is prevented. In sum, the electric discharge resistantcharacteristics in the entire insulating layer are improved. Moreover,an insulating layer suited to conditions for practical use can be formedby changing a ratio of thickness between a plurality of organicinsulating layers.

In the metal base board and electronic equipment using the same therein,the insulating section comprises an organic insulating material withmultiple different types of flaky inorganic fillers each made of adifferent raw material, surfaces of the flaky inorganic filler materialsare aligned basically in parallel with the surfaces of the circuitconductor section and the base section so that the filler materials canbe stacked in a stratified state. For this reason, even if electricdischarge occurs in a section of the circuit conductor along the surfaceor in a void within the insulating layer and the organic insulatingmaterial is degraded due to the electric discharge, the degradation dueto electric discharge is suppressed in the section with the flakyinorganic fillers added therein, or the degradation due to electricdischarge itself is prevented with the electric discharge resistantcharacteristics in the entire insulating layer improved. Furthermore,properties corresponding to the type of filler material can be given tothe insulating layer, and for these reasons an insulation layer moresuited to conditions for practical use can be formed.

In the board according to the present invention and electronic equipmentusing the same, the insulating section comprises a plurality ofinsulating layers, each layer comprises an organic insulating layer withflaky inorganic fillers each having a different particle diameter addedtherein respectively, surfaces of each flaky inorganic fillers arealigned basically in parallel with the surfaces of the circuit conductorsection and the base section so that the fillers can be stacked in astratified state. As a result, even if electric discharge occurs in asection of the circuit conductor along the surface or in a void withinthe insulating layer and the organic insulating material is degraded dueto the electric discharge, the degradation due to electric discharge issuppressed in the section with the flaky inorganic fillers addedtherein, or the degradation due to electric discharge itself isprevented with the electric discharge resistant characteristics improvedin the entire surface. Moreover, because properties corresponding to thetype of filler material can be given to the insulation layer, aninsulating layer suited to conditions for practical use can be formed.

Other objects and references of this invention will become understoodfrom the following description with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating configuration of a metal base boardaccording to the present invention;

FIG. 2 is a drawing illustrating a metal base board according to thepresent invention;

FIG. 3 is a view illustrating a metal base board according to thepresent invention;

FIG. 4 is a view illustrating a metal base board according to thepresent invention;

FIG. 5 is a view illustrating a metal base board according to thepresent invention;

FIG. 6 is a view illustrating a metal base board according to thepresent invention; and

FIG. 7 is a view illustrating a metal base board according to thepresent invention.

DESCRIPTION OF THE EMBODIMENTS

Now description is made hereinafter for embodiments of metal base boardsaccording to the present invention and electronic equipment using thesame with reference to the related drawings. As shown in FIG. 1, a metalbase board 10 has a circuit conductor 3 via an insulating layer 2 on ametal plate 1 with flaky inorganic fillers 4, the flakes thereof havinga form like a flat plate, added in the insulating layer 2. Such amaterial as Fe, Al, Cu, SUS, or silicon steel, for example, can be usedas a material for the metal plate 1, but Cu is used in this embodiment.As a material for the circuit conductor 3, such a material as Al or Cu,for example, can be used, but Cu is used in this embodiment.

In the insulating layer, an organic insulating material with flakyinorganic fillers added therein is used, and in this embodiment resinwith flaky inorganic fillers each comprising an alumina filler (marketedby Showa Keikinzoku under the name of Low soda alumina L-13PC) addedtherein by 80 weight % against the resin is used. The flaky inorganicfillers are added in a quantity of 80% against the resin because, if theflaky inorganic fillers are mixed too significantly in the resin, theviscosity of the resin increases to much and the workability thusbecomes low.

Specifically, epoxy resin and diciandiamido curing agent are mainly usedas an organic insulating material, and alumina flakes each having a sizeapproximately 0.06 mm and the thickness from 0.002 to 0.003 mm are usedas the flaky inorganic fillers 4. Furthermore the thickness of aninsulating layer in a board is 0.12 mm, and about 20 to 30 flake layersare formed in the insulating layer 2.

The flakes in the flaky inorganic fillers may be circular, rectangular,polygonal, or of other form, so long as the materials are flat.Furthermore flat surfaces of each inorganic filler flake 4 in theorganic insulating material are generally aligned in parallel with thesurfaces of the circuit conductor 3 and the metal plate 1 so that theflaky inorganic fillers can be stacked in a stratified state. Namely,the flaky inorganic fillers are added so that as seen by the paths 11,any vector from a given point on the circuit conductor 3 to the metalplate 1 in the direction vertical to the circuit conductor 3 willintersect some of the flaky inorganic fillers.

In the insulating layer 2, various methods may be employed for aligningthe flaky inorganic fillers in one direction. For instance, (1) powderof flaky inorganic fillers, each flake having a form like a flat plate,is added in an organic insulating material having a low viscosity, themixture is applied several times to form a thin layer, and then the thinlayer is cured; (2) the flaky inorganic fillers are aligned in onedirection by making use of gravity; or (3) powder of flaky inorganicfillers, each flake having a form like a flat plate, is disposed on ametal plate, and then a layer of organic insulating material is appliedso that it becomes impregnated with the flakes. In this embodiment,powder of flaky inorganic fillers 4, each flake having a form like aflat plate, are added to an organic insulating material having a lowviscosity and is applied several times in a form of thin layers (10 to20 μ/1 time×10 times), and the thin layers are cured by loading pressureto and heating the thin layers in a vacuum.

Testing for the electric discharge resistant characteristics of theboard obtained according to the method described above was carried out.In this testing for the resistance against electric discharge, sampleswere obtained by etching a 5 mm (width)×50 mm (length) conductivepattern at a center of a metal base board having outer dimensions of 200mm (length)×150 mm (width). The thickness of the metal plate was 2 mm,the thickness of the organic polymeric insulating layer was 0.12 mm, andthe thickness of the conductive layer was 0.1 mm.

In the test, industrial power (AC 2 kV and AC 3 kV, 60 Hz) was appliedbetween the conductive pattern and the metal plate of the metal baseboard at a peripheral temperature of 100° C. The time required until theinsulation was broken was measured. When an AC voltage is applied, airnear the surface of the conductive pattern will result in a violentdischarge of electricity. Results obtained in this test are shown inTable 1.

                  TABLE 1                                                         ______________________________________                                                    Thickness                                                                             Time until insul.                                                     of Insulting                                                                          Broken (Hr)                                                           Layer (μm)                                                                         2 k V loaded                                                                             3 k V loaded                                   ______________________________________                                        Flaky Inorganic Filler                                                                      120       9500       4300                                       Materials                                                                     Granular Inorganic Filler                                                                   120       250        80                                         Materials                                                                     ______________________________________                                    

As clearly shown in Table 1, the metal base board in which flakyinorganic fillers are used has far superior electric discharge resistantcharacteristics as compared to that of a conventional product (a metalbase board in which a granular inorganic filler material is used).Namely, in a metal base board 10 having the configuration as describedabove, an electric discharge generated in the side of the circuitconductor 3 degrades the insulating layer 2, and the degradationproceeds up to the section with the flaky inorganic fillers 4 added inthe insulating layer 2. As described above, surfaces of flaky inorganicfillers 4 each constituting a portion of the insulating layer 2 arealigned basically in parallel with the surfaces of the circuit conductor3 and the metal plate 1, and in addition the fillers are stacked instratified layers. With this structure, although electric dischargeoccurs in the flaky inorganic filler material section, progress ofdegradation is very slow because the flaky inorganic fillers 4 provideexcellent electric discharge resistant characteristics. As a result, theperformance as shown in Table 1 is provided. Namely, the metal baseboard having a circuit conductor 3 formed over the insulating layer 2 onthe surface of the metal plate 1, in which the flaky inorganic fillers 4are added in the insulating layer 2 with the surfaces of each flake ofthe flaky inorganic filler material 4 aligned generally in parallel withthe surfaces of the circuit conductor 3 and the metallic body 1 and thealigned flakes 7 inorganic filler stacked in a stratified state, hassuperior electric discharge resistant characteristics as compared tothat of a conventional type of board.

Semiconductor devices, such as transistors, resistors, and capacitorsfor power control which generate a large quantity of heat, are mountedon a metal base board having the configuration as described above tobuild a variety of electronic equipment.

Now description is made hereinafter of a second embodiment of thepresent invention. As shown in FIG. 2, a metal base board 20 has thecircuit conductor 3 via the insulating layer 2 on the metal plate 1 withflaky inorganic filler materials 4 and a granular inorganic fillermaterial added in the insulating layer 2. In this embodiment, a resinwith flaky alumina inorganic fillers (marketed by Showa Keikinzoku underthe name of Low soda alumina AL-13PC) by about 40 weight % against theweight of resin and granular inorganic filler materials (Alumina fillerhaving a spheric form produced by Showa Denko) by about 50 weight %against the weight of resin added therein, respectively, is used.

In the organic insulating material with the flaky inorganic fillers 4and the granular inorganic filler materials 5 added therein, thesurfaces of the flaky inorganic fillers are aligned basically inparallel with the surfaces of the circuit conductor 3 and the metalplate 1, and are stacked in a stratified state. Namely, any vector inthe insulating section 2 from a given point on the circuit conductor 3to the metal plate 1 in a direction vertical to the circuit conductor 3,as seen by the paths 21, will intersect flakes in the flaky inorganicfillers 4.

Various methods can be employed for aligning the flaky inorganic fillers4 in one direction. However, in this embodiment, the flaky inorganicfiller 4 and a granular inorganic filler material 5 are added in powderform to an organic insulating material having a low viscosity. Themixture is applied several times in a form of thin layers. Then, thethin layer is cured in a vacuum by applying pressure and heat. As aresult, the viscosity of the resin could be reduced to about 70% ascompared to a case where only the flaky inorganic fillers 4 described inrelation to the first embodiment was added, and also the workabilitycould be improved. Accordingly, the viscosity of an insulating materialcan be reduced and also the linear thermal expansion coefficient as wellas the heat conductivity can easily be adjusted in a manufacturingprocess.

Testing for the electric discharge resistant characteristics of theboard obtained according to the method as described above was carriedout in the same procedure as that in the first embodiment. The result ofthis testing are as shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                    Thickness                                                                             Time until insul.                                                     of Insulting                                                                          Broken (Hr)                                                           Layer (μm)                                                                         2 k V loaded                                                                             3 k V loaded                                   ______________________________________                                        Flaky + Granular Inorganic                                                                  120       7800       3080                                       Materials                                                                     Granular Filler Materials                                                                   120       250        80                                         ______________________________________                                    

As clearly shown in Table 2 above, for a metal base board with flakyalumina filler materials and granular alumina filler materials added inan organic insulating material thereof, the electric discharge resistantcharacteristics are substantially better as compared to that in theconventional products. The mechanism for improving the electricdischarge resistant characteristics in this embodiment is basically thesame as that in the first embodiment.

Electronic equipment are constructed by packaging semiconductor devicesfor power control, such as transistors, resistors, and capacitors, whichgenerate a quantity of heat, on a metal base board having theconfiguration as described above.

Now description is made for a third embodiment of the present invention.As shown in FIG. 3, a metal base board has the circuit conductor 3formed via the insulating layer 2 on the upper surface of the metalplate 1, the insulating layer 2 comprises a first insulating layer 2aand a second insulating layer 2b, the first insulating layer 2a isfilled with the flaky inorganic filler materials 4, and the secondinsulating layer 2b is filled with the granular filler materials 5.

In the first insulating layer 2a, an organic insulating material such asan epoxy resin with the flaky inorganic filler materials 4 added thereinis used. In this embodiment, alumina filler flaky inorganic fillers areadded by about 80% against the weight of resin (available for ShowaKeikinzoku under the name of Low soda alumina AL-13 PC).

In the second insulating layer 2b, an organic insulating material suchas an epoxy resin with a granular inorganic filler material added isused. In this embodiment, alumina filler granular inorganic fillers(Ultra-granular alumina, spheric product sold by Showa Denko) are addedby about 90 weight % against the weight of resin.

Also, in the metal base board according to this embodiment, the firstinsulating layer 2a made of an organic insulating material with theflaky inorganic filler materials 4 added therein is formed in theinsulating layer at the side adjacent the circuit board 3. Thisarrangement is effective for improving the electric discharge resistantcharacteristics thereof. In a manufacturing process, the secondinsulating layer 2b with the granular inorganic filler materials 5 addedtherein is first formed on the metal plate 1, then the first insulatinglayer 2a with the flaky inorganic fillers 4 added therein is formed, andfinally the circuit conductor 3 is adhered.

Surfaces of the flaky inorganic fillers 4 added in the first insulatinglayer 2a are aligned basically in parallel with the surfaces of thecircuit conductor 3 and the metal plate 1 in the organic insulatingmaterial, and also the flaky inorganic fillers are stacked in astratified state. Namely, the flaky inorganic fillers are added so thatany vector from a given point on the circuit conductor 3 to the metalplate 1 in the direction vertical to the circuit conductor 3, as seen inpaths 31, will intersect a flake in the flaky inorganic filler in theinsulating section 2.

Various methods can be employed for aligning inorganic filler materialsin one direction. In this embodiment, at first a powder of granularfiller material 5 is added to an organic insulating material having alow viscosity, and the mixture is applied several times in a form ofthin layer, which is cured to the thickness of 0.06 mm by applyingpressure and heat. Furthermore, powder of the flaky inorganic fillermaterial 4 is added in an organic insulating material having a lowviscosity, and the mixture is applied several times in a form of thinlayers, which are cured to the thickness of 0.06 mm by applying pressureand heat in a vacuum. Thus, an insulating layer having the totalthickness of 0.12 mm is formed and the circuit conductor 3 is adheredthereto.

Testing for the electric discharge resistant characteristics of theboard obtained as described above was carried out according to the sameprocedure as in the first and second embodiments above. The results ofthis testing are as shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                     Thickness                                                                             Time until insul.                                                     of Insulting                                                                          Broken (Hr)                                                           Layer (μm)                                                                         2 k V loaded                                                                            3 k V loaded                                   ______________________________________                                        2-layer Structure Based on the                                                               120       8800      3800                                       Flaky and Granular Inorganic                                                  Filler Materials                                                              Granular Inorganic Filler                                                                    120       250       80                                         Materials                                                                     ______________________________________                                    

As clearly shown in Table 3, the metal base board according to thisembodiment is far superior in the electric discharge resistantcharacteristics as compared to that of the conventional products. Themechanism for improving the electric discharge resistant characteristicsis basically the same as that in the first embodiment. However, as thethickness of the first insulating layer 2a with the flaky inorganicfiller materials added therein is smaller as compared to that of theinsulating layer in the first embodiment described above, the time untilbreakage of insulation becomes shorter.

Electronic equipment are built by packaging semiconductor devices forpower control as transistors, resistors, and capacitors which generate aquantity of heat on the metal base board having the configuration asdescribed above.

Now description is made hereinafter for a fourth embodiment of thepresent invention. As shown in FIG. 4, a metal base board 40 has thecircuit conductor 3 disposed the insulating layer 2 on the upper surfaceof the metal plate 1. The insulating layer 2 comprises the firstinsulating layer 2a and the second insulating layer 2b. The flakyinorganic fillers 4 are added in the first insulating layer 2a, andflaky inorganic fillers 6 having the average size different from that ofthe flaky inorganic fillers 4 added in the first insulating layer 2a areadded in the second insulating layer 2b.

In the metal base board according to this embodiment, disposing thefirst insulating layer 2a made of an organic insulating material withthe flaky inorganic filler materials 4 having a relatively large averagesize adjacent the circuit board 3 is effective in improving the electricdischarge resistant characteristics of the assembly. Accordingly, atfirst the second insulating layer 2b with the flaky inorganic fillers 7having a short average dimension is formed on the metal plate 1. Thenthe first insulating layer 2a with the flaky inorganic fillers 4 havinga lengthy average size added therein is formed there on. Finally thecircuit conductor 3 is adhered.

Surfaces of the flaky inorganic fillers 4 in the insulating layer 2a andthe flaky inorganic fillers 6 in the second insulating layer 2b arealigned basically in parallel with the surfaces of the circuitconductors 3 and the metal plate 1, and the fillers are stacked in astratified state. Accordingly, any vector in the insulating section froma given point on the circuit conductor 3 to the metal plate 1 in thedirection vertical to the circuit conductor 3, as seen along paths 41will intersect flakes in the flaky inorganic filler materials 4 and 6.

Also in this configuration, the electric discharge resistantcharacteristics are far better than that of conventional products. Themechanism for improving the electric discharge resistant characteristicsin this embodiment and the method of aligning the direction of theinorganic filler materials are basically the same as those in the firstand the third embodiments.

SemiConductor devices for power control such as transistors, resistors,and capacitors, which generates a large quantity of heat, are mounted onthe metal base board having the construction as described above to buildelectronic equipment.

Now description is made for a fifth embodiment of the present invention.As shown in FIG. 5, a metal base board 50 has the circuit conductor 3formed via the insulating layer 2 on the upper surface of the metalplate 1. Flaky inorganic filler material 4 of a first type and a secondtype of flaky inorganic material are disposed in mixed form in theinsulating layer 2. In an organic insulating material with the randommix of flaky inorganic fillers 4 and the flaky inorganic fillers 7 addedtherein, surfaces of the flaky inorganic filler materials 4 and 7 arealigned basically in parallel with the surfaces of the circuit conductor3 and the metal plate 1, and the inorganic filler flakes are stacked ina stratified state. Namely any vector from a given point on the circuitconductor 3 to the metal plate 1 in the direction vertical to thecircuit conductor 3 as seen from paths 51 will intersect with flakes ofthe flaky inorganic fillers 4 and 7. Any of, for instance, mica, boronnitride, flaky glass, and flaky alumina is used as a raw material forthe flaky inorganic fillers 4 and 7 described above.

Also in this configuration, the electric discharge resistantcharacteristics are far better than that of conventional products. Themechanism for improving the electric discharge resistant characteristicsin this embodiment and the method of aligning the direction of theinorganic fillers in one direction are basically the same as those inthe first embodiments.

Semiconductor devices for power control such as transistors, resistors,and capacitors, which generates a large quantity of heat, are mounted onthe metal base board having the construction as described above to buildelectronic equipment.

Next description is made of a sixth embodiment of the present invention.As shown in FIG. 6, a metal base board 60 has the circuit conductor 3formed via the insulating layer on the upper surface of the metalplate 1. The insulating layer 2 comprises the first insulating layer 2aand the second insulating layer 2b. The flaky inorganic fillers 4 areadded in the first insulating layer 2a, and flaky inorganic fillers 8made of a raw material different from that of the flaky inorganicfillers 4 added in the first insulating layer 2a are added in the secondinsulating layer 2b. With this arrangement, any vector from a givenpoint on the circuit conductor 3 to the metal plate 1 in the directionvertical to the circuit conductor 3, as seen from paths 61 willintersect a flake of the flaky inorganic fillers 4 and 8. Any of, forinstance, mica, boron nitride, flaky glass, and flaky alumina is used asa raw material for the flaky inorganic fillers 4 and 8 described above.

Surfaces of the flaky inorganic filler materials 4 in the insulatinglayer 2a and the flaky inorganic fillers 8 in the second insulatinglayer 2b are aligned basically in parallel with the surfaces of thecircuit conductors 3 and the metal plate 1, and the fillers are stackedin a stratified state. Also in this configuration, the electricdischarge resistant characteristics is far better than that ofconventional products like in each of the embodiments described above.The mechanism for improving the electric discharge resistantcharacteristics in this embodiment and the method of aligning directionof the inorganic fillers in one direction is basically the same as thosein the first and the third embodiments.

Semiconductor devices for power control such as transistors, resistors,and capacitors, which generates a large quantity of heat, are mounted onthe metal base board having the construction as described above to buildelectronic equipment.

It should be noted that the metal base boards according to the presentinvention are not limited to those described in relation to the aboveembodiments and can be modified within the spirit of the presentinvention. In the embodiments described above, a circuit conductor isformed on only one side of the metal plate, but circuit conductors maybe formed via an insulating layer on two sides of the metal plate. Alsothe metal base board shown in above third embodiment, fourth embodimentand sixth embodiment is comprised by two layers including an insulatinglayer, but the metal base board according to the present invention arenot limited to two layer structure, and can be modified to the metalbase board having plural layers.

As described above, in a metal base board and electronic equipment usingthe same therein, an insulating section comprises an organic insulatingmaterial with flaky inorganic fillers added therein, and the flakyinorganic fillers are stacked in a stratified state, so that, even ifthe insulating layer is degraded due to electric discharge, thedegradation due to electric discharge is prevented by the flakyinorganic fillers added therein, and for this reason the electricdischarge resistant characteristics is improved.

Also, in the metal base board according to the present invention andelectronic equipment using the same therein, the insulating sectioncomprises an organic insulating material with flaky inorganic fillersand a granular inorganic filler material added therein, and the flakyinorganic fillers are stacked in a stratified state. As a result, evenif the insulating layer is degraded due to an electric discharge, thedegradation due to electric discharge is prevented by the section withthe flaky inorganic fillers added. For this reason, the electricdischarge resistance characteristics are improved with the workabilityalso raised. Further, the required performance can be satisfied bymixing inorganic fillers having various characteristics without beingrestricted by the flaky inorganic fillers.

Also in the metal base board according to the present invention andelectronic equipment using the same therein, the insulating sectioncomprises a first insulating layer made of an organic insulatingmaterial with flaky inorganic fillers added therein and a secondinsulating layer made of an organic insulating material with granularinorganic filler material added therein, and the flaky inorganic fillermaterials are stacked in a stratified state. As a result, even if theinsulating layer is degraded due to electric discharge, the degradationdue to electric discharge is prevented by the flaky inorganic fillermaterials. For this reason, the electric discharge resistancecharacteristics is improved with the workability also raised, and arequired performance can be satisfied by mixing inorganic fillermaterials having various characteristics without being restricted by theflaky inorganic filler materials.

Also in the metal base board according to the present invention andelectronic equipment using the same therein, the insulating sectioncomprises an organic insulating material with flaky inorganic fillersmade of a plurality of different types of materials added therein, andthe flaky inorganic fillers are stacked in a stratified state.Accordingly, even if the insulating layer is degraded due to electricdischarge, the degradation due to electric discharge is prevented by thesection with the flaky inorganic fillers added. For this reason theelectric discharge resistance characteristics are improved with theworkability also raised, and a required performance can be satisfied bymixing inorganic filler materials having various characteristics withoutbeing restricted by the flaky inorganic fillers.

Also in the metal base board according to the present invention andelectronic equipment using the same therein, the insulating sectioncomprises a plurality of insulating layers, the insulating layerscomprise an organic insulating material with flaky inorganic fillermaterials made of a different material respectively added therein, andthe flaky inorganic fillers are stacked in a stratified state.Accordingly, even if the insulating layer is degraded due to electricdischarge, the degradation due to electric discharge is prevented by thesection with the flaky inorganic fillers added therein. For this reasonthe electric discharge resistance characteristics are improved with theworkability also raised, and a required performance can be satisfied bymixing inorganic filler materials having various characteristics withoutbeing restricted by the flaky inorganic filler materials.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. A metal baseboard comprising:a metallic basesection; a circuit conductor section; and an insulating section providedbetween said circuit conductor section and said base section; saidinsulating section comprising a plurality of insulating layers, at leasttwo of said insulating layers being adjacent to each other andcomprising an organic insulating material with respective flakyinorganic filler material made of a different raw material respectivelyadded therein, said flaky inorganic fillers being disposed in saidinsulating section in a substantially parallel relation to each other.2. Electronic equipment, comprising:a metal base board having a metallicbase section, a circuit conductor section, and an insulating sectionprovided between said circuit conductor section and said base section;said insulating section comprising a plurality of insulating layers, atleast two of said insulating layers being adjacent to each other andcomprising an organic insulating material with respective flakyinorganic fillers made of a different raw material respectively addedtherein, said flaky inorganic fillers being disposed in said insulatingsection in a substantially parallel relation to each other.